Reciprocity. Learned form one of my favorite english teachers who, while we were all writing short stories or something (him included), exclaimed out loud that he couldn’t remember the word and tried to give us a definition. But it is such an akward word that he could barely define it–and none of us knew it anyway. He came in the next day and prouldy spelt out reciprocity on the board.
Oddly enough, I still have a hard time remembering the word. It usually goes like “what’s that word that’s so hard to remeber? … ah! I can’t remember!”
It’s almost impossible to actually use in a non-technical sentence. My favorite use (which I basically made up) is “This reciprocal saw reciprocates with greater reciprocity than the old model.”